This invention concerns a cement for surgical purposes, a method for stabilizing a brushite cement used as temporary bone replacement material and the temporary bone replacement material obtained by said method.
A number of such hydraulic cements based on calcium phosphates for use in surgery are known in the prior art; they are prepared from two components (powder/liquid) by mixing them intraoperatively and applying them in pasteous consistency to the appropriate site where they harden in situ. The disadvantages of the prior art hydraulic cements based calcium phosphates are:
a) impracticable short setting times which do not allow their use for elaborate surgical procedures;
b) poor injectability, i.e. the fresh cement paste tends to clog the injection needle, and/or disintegrates in contact with physiological liquids, which prevents its implantation by minimal invasive surgery procedures;
c) low compacity, i.e. current hydraulic cements need larger amounts of mixing water in order to have them injectable or to confer them a convenient setting time, which results in very low ultimate mechanical strength after hardening; and
d) too fast resorption, i.e. the cement resorbs faster than the bone growth rate, resulting in a non-osseous gap between bone and cement which is detrimental to the mechanical properties of the cement.
In the U.S. Pat. No. 4,880,610 CONSTANTZ a method is disclosed for making an in situ calcium phosphate mineral composition by combining water-free phosphoric acid crystals with a calcium source which leads to a hydroxyapatite. It is clear that the use of 100% phosphoric acid in the operating room and the application of a paste containing 100% phosphoric acid in the human body must be considered a not ideal procedure which requires improvement.
In U.S. Pat. No. 5,129,905 CONSTANTZxe2x80x94in order to reduce the problemxe2x80x94phosphoric acid crystals were replaced by monocalcium phosphate monohydrate (MCPM) or monocalcium phosphate (MCP). However, the goal was again to obtain a hydroxyapatite material, which has a long resorption period. A long resorption period is not commensurate to the rate of the bone remodelling. The disadvantage of prolonged resorption is that the bone treated by cement will remain for a prolonged time in abnormal biomechanical situation, which may develop secondary post-operational problems. Furthermore, the unresorbed cement may still break down in pieces or fragments after prolonged mechanical loading, which increases the probability of post-operational complications, e.g. aseptic inflammatory reactions. The resorption rate of the ideal cement should match as closely as possible the spontaneous rate of new bone formation which is around 20 micrometers per day. A too fast resorption rate is also not wanted. Certain studies done with plaster of Paris and calcium phosphate cement have shown that the resorption rate is faster than the bone growth rate, leading to a gap between bone and cement. This is obviously detrimental to the mechanical stability of the defect site.
From U.S. Pat. No. 5,605,713 BOLTONG a calcium phosphate composition is known which may contain (among others) xcex2-TCP, MCPM, water and magnesium salts. However, the invention is limited to pH values in the range of 6.5 to 8.0, range in which brushite does not precipitate. A pH below 6,5 preferably below 4 is needed to obtain brushite. In the pH range of 6.5 to 8.0, octocalcium phosphate and hydroxyapatite are the phases precipitating. However, these phases are much less soluble than brushite at neutral pH and thus lead to too slow resorption rates.
From PCT/EP98/06330 a calcium phosphate composition is known which contains brushite (dicalcium phosphate dihydrate; CaHPO4.2H2O) as end-product of the setting reaction. This cement has however a too fast resorption rate in vivo, leading to mechanical instabilities and inflammatory reactions.
The invention as claimed aims at solving the above described problems. The present invention provides a cement for surgical purposes, a method for producing a temporary bone replacement material as defined and a temporary bone replacement material.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to an forming part of this disclosure. For the better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be done to the accompanying examples in which preferred embodiments of the invention are illustrated in detail.
The first component of the cement according to The invention comprises a basic calcium phosphate, preferably xcex2-tricalcium phosphate [xcex2-Ca3(PO4)2; xcex2-TCP], xcex1-tricalcium phosphate [xcex1-Ca3(PO4)2; xcex1-TCP], tetracalcium phopshate Ca4(PO4)2O; TetCP], oxyapatite Ca10(PO4)6O; OXA], hydroxyapatite [Ca5(PO4)3OH; HA], or calcium-deficient hydroxyapatite [Ca10xe2x88x92x(HPO4)x(PO4)6xe2x88x92x(OH)2xe2x88x92x; CDHA] powder. It can also be a mixture of two or three of the latter compounds. xcex2-TCP is the preferred compound.
The second component of the cement according to the invention comprises an acidic calcium phosphate, preferably monocalcium phosphate monohydrate [Ca(H2PO4)2xc2x7H2O; MCPM], monocalcium phosphate [Ca(H2PO4)2; MCP], or phosphoric acid [H3PO4] powder. It can also be a mixture of two or three of the latter compounds. MCPM is the preferred compound.
The third component of the cement according to the invention comprises water.
The fourth component of the cement according to the invention comprises a source of magnesium used to stabilize the end-product of the setting reaction.
The setting reaction is characterized by several partial reaction: dissolution of the first component, dissolution of the second component and precipitation of the end-product of the setting reaction, i.e. brushite (dicalcium phosphate dihydrate; CaHPO4xc2x7H2O). Normally, the dissolution reaction of the second component is much faster than that of the first component. As the second component is acidic, the cement paste reaches pH values of 2 to 4 depending on the cement composition and particle size distribution. When the second component is completely dissolved, the ongoing dissolution of the first component, which is basic, provokes an increase of the pH value of the cement paste. However, the pH of the cement according to the invention at the end of the setting reaction is always in the range of 2 to 6. It is also in this range that brushite preferentially precipitates. At higher pH values (6 and higher), brushite does not precipitate: octocalcium phosphate and hydroxyapatite are the phases precipitating. However, these phases are much less soluble than brushite at neutral pH and would thus lead to too slow resorption rates.
The particular size distribution and the mean specific surface area of the solid components has a large influence on the physico-chemical properties of the cement, in particular the setting time, the mechanical properties, and the workability. Generally speaking, powders with a high specific surface area lead to short setting times, high mechanical-properties, and good workability. However, this rule is no more valid when the powders are agglomerated: a large amount of mixing liquid is required to water the powders, hence leading to poor mechanical properties. Therefore, powders should preferably be desagglomerated.
A good workability depends very much on the application. In some cases, a rather liquid paste is desired (reinforcement of osteoporotic bones). In other cases, a very thick paste may be the most adequate (e.g. plastic surgery). A powder with a small average particle size possess only a very small range in which a paste can be formed with water the paste is either solid and breakable, or very liquid. Generally, a thick paste is preferred, because the paste is more easily workable and remains stable upon contact with body fluids. So a paste containing powders with a large average particle size is normally chosen. A typical range for the mean particle size of the powders is 0.1 to 100 micrometers.
The solubility of the different solid components has a large influence on the cement setting time. If the first component is very soluble, the setting reaction is fast. If the first component is poorly-soluble, the setting reaction tends to be slow. For example, the use of the very soluble xcex1-TCP or TetCP powder as first component leads to very short setting times. The use of the rather soluble xcex2-TCP powder as first component leads to short setting times. Finally the use of the rather insoluble HA, CDHA or OXA powder leads to long setting times. So, in order to obtain a setting time in the order of 5 to 20 minutes, it is desirable to use a xcex2-TCP powder with a small specific surface area, and a CDHA or HA powder with a large specific surface area. Assuming that the powders are desagglomerated, a typical range for the mean particle size of the powders is 0.1 to 1 micrometers for CDHA , OXA, or HA powder, 1 to 10 micrometers for xcex2-TCP powder, and 5 to 100 micrometers for xcex1-TCP or TetCP powder.
Despite the use of large xcex1-TCP , TetCP, and/or xcex2-TCP particles, the setting time of a cement containing xcex1-TCP, TetCP, and/or xcex2-TCP as first component is normally too short. Setting time of the cement according to this invention as measured at 25xc2x0 C. should preferably be at least 1 minute, typically at least 2 minutes and preferably at least 5 minutes. So, a setting rate controller is normally used. It is chosen from the group of sodium pyrophosphate, potassium pyrophosphate, sodium acetate, potassium acetate, sodium citrate, potassium citrate, sodium phosphocitrate, potassium phosphocitrate, sodium sulfate, potassium sulfate, calcium sulphate hemihydrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, magnesium sulfate and sodium or potassium biphosphonate. The setting rate controller can be added either pre-dissolved in the third component or as a solid (powder). However, in the latter case, the setting rate; controller must be very soluble, so that the solid dissolves almost instantaneously upon contact with water. Sodium pyrophosphate and sodium sulfate are normally the preferred setting rate controllers. If HA, OXA, or COHA are used as first component, the setting time is normally too long. The setting time can be decreased by adding appropriate setting rate controllers. Efficient setting rate controllers are compounds containing orthophosphate ions such as sodium, magnesium or potassium orthophosphate salts, or phosphoric acid.
The third component comprising water may further comprise phosphoric acid (OPA) and/or sulfuric acid (SA), which again take the function of a setting rate controller and also lead to an improved microstructure of the final brushite crystals.
To control the resorption rate of the cement, granules having an average diameter which is larger than the average diameter of said first component can be added. This leads to conglomerate structure of the finally set cement, whereby the granules are embedded in the brushite matrix formed by the setting process. The average particle diameter of the latter granules should be at least two times larger, preferably at least 10 times larger compared with the average diameter of the particles of the first component. The average particle diameter of said granules should be in the range of 50 to 2000 micrometers. Preferably, it should be in the range of 100 to 500 micrometers, preferably in the range of 200 to 350 micrometers. The granules may consist of calcium phosphates, e.g. xcex1-TCP, TetCP, OXA, xcex2-TCP, HA, CDHA, biphasic calcium phosphate (BCP), gypsum, bioglass, and polymers, e.g. lactides, polysaccharides, collagen, proteins. The preferred composition for these granules is xcex2-TCP. The advantage of using granules is the differential degradation of such a cement. The matrix of the cement is degraded faster or slower than the residual granulates. This is particularly useful for the application in the osteoporose field or for ridge reconstruction of the jaw, where a slower degrading granule, e.g. made from hydroxyapatite or BCP is desired. The use of a fast resorbable granule (e.g. gypsum) may allow the obtention of a macroporous cement structure after a short implantation time.
The volume VL of the third component should preferably be equal or superior than the volume VT=(WMCPAxc3x970.615+WMCPMxc3x970.5+WOPAxc3x971.102+WSAxc3x971.101) ml/g of the second component, where WMCPA, WMCPM, WOPA, and WSA are the weight of MCP, MCPM, phosphoric acid and sulfuric acid, respectively. The volume VL is typically in the range of 0.5xc3x97VTxe2x89xa6VLxe2x89xa610xc3x97VT, preferably in the range of 1.2xc3x97VT+xe2x89xa6VLxe2x89xa62.5xc3x97VT. The amount of mixing liquid (third component) has a strong influence on the physico-chemical properties of the cement, in particular the setting time, and the mechanical properties. The setting time and the cement porosity increase with an increase of VL. As the mechanical properties are decreased by an increase of porosity, an optimum for VL can be chosen regarding setting time and mechanical properties.
To optimize the cement compatibility in vivo, the cement should contain an excess of basic components, i.e. an excess of the first component compared with the second component. In other words, the Ca:P molar ratio of the cement must be superior or equal to 1.0. The Ca:P molar ratio can be written (assuming that x=1 in CDHA (Ca10xe2x88x92x(HPO4)x(PO4)6xe2x88x92x(OH)2xe2x88x92x)):
Ca:P ratio=(WMCP/236+WMCPM/252+3xc3x97WTCP/310+4xc3x97WTetTCP/366+10xc3x97WOXA/986+5xc3x97WHA/502+9xc3x97WCDHA/948)/(2xc3x97WMCP/236+2xc3x97WMCPM/252+2xc3x97WTCP/310xc3x972xc3x97WTetCP/366+6xc3x97WOXA/986+3xc3x97WHA/502+6xc3x97WCDHA/948+WOHA/98) where WMCP, WMCPM, WTCP, WHA, WCDHA and WOHA are the weight of MCP, MCPM, xcex1 or xcex2-TCP, TetCP, OXA, HA, CDHA and phosphoric acid respectively. The preferred Ca:P molar ratio lies in the range of 1.00 to 1.67, preferably in the range of 1.05 to 1.30.
One of the four components may further comprise a biodegradable polymer for controlling the consistency of the cement paste resulting from mixing of the two components, and its cohesion in physiological liquids. There are two goals in controlling the cement consistency: (i) by increasing the viscosity of the mixing liquid, the paste becomes less sensitive to filter-pressing (no demixing during injection); and (ii) by increasing the viscosity of the mixing liquid, the viscosity of the cement paste is increased and the cement paste does not decompose when put into an aqueous solution.
The biodegradable polymer may be selected from the group of hyaluronic acid, hyaluronatesalts, dextran, alginate, hydroxypropylmethyl cellulose, chitosan, xanthan gum, agarose, polyethylene glycols (PEG), polyhydroxyethylenemethacrytats (HEMA), synthetic and natural proteins, or collagen.
The cement may further comprise pharmaceutically or physiologically active substances, preferably selected from the group of antibiotics, anti-inflammatory drugs, anti-cancer drugs, peptides, and proteins such as growth factors. The antibiotic is preferably a gentamicin or a gentamicin salt, typically gentamicin sulfate. Other gentamicin salts can be used provided their solubility is in the range of 100 to 2500 mg/L.
The antibiotic is selected from the group of aminoglycosides, vancomicins, gentamicins or salts thereof, preferably gentamicin sulfate or gentamicin crobefat.
The cements according to the invention may be used as bons substitute in dental and maxillofacial surgery (alveolar ridge reconstruction, dental socket filling), for orthopaedic applications (bone fracture repair, bone augmentation) and for local drug delivery (antibiotics, anti-inflammatory and anti-cancer drugs).
The third component of the cement may further comprise a hydrophobic liquid that can act either as a lubricant or as a pore forming agent. In the latter case, the cement mixture is stirred mechanically until an emulsion is obtained. The paste can then be injected. After setting, the hydrophobic liquid is entrapped in the cement matrix, hence forming pores. The emulsion can be stabilized by means of an emulsifier. The hydrophobic liquid and the emulsifier should be preferably chosen for in vivo implantation. Compounds of choice are all natural products.
The fourth component of the cement is taken from the group of MgO, MgO2, Mg(OH)2, MgHPO4, MgHPO4xc2x73H2O, MgHPO4xc2x77H2O, Mg3(PO4)2, Mg3(PO4)2xc2x74H2O, Mg3(PO4)2xc2x78H2O, Mg,(PO4)2xc2x722H2O, MgCO3, MgCO3xc2x73H2O, MgCO3xc2x75H2O, 3MgCO3xc2x7Mg(OH)2xc2x73H2O, MgCO3Mg(OH)2xc2x73H2O, Mg(C3H5O3)2xc2x73H2O, MgC2O4xc2x72H2O, MgC4H4O6xc2x75H2O, Mg(C4H4O6)2xc2x74H2O, MgCO3xc2x7CaCO3, Mg2P2O7, Mg(C12H23O2)2xc2x72H2O, Mg(C14H27O2)2, Mg(C18BH33O2)2, Mg(C18H35O2)2xc2x7The amount of the fourth component should be comprised in the range of 0.001 to 60% w/w, more precisely in the range of 1 to 20% w/w, preferably in the range of 2 to 5% w/w. The magnesium salt should not be too soluble to prevent a fast release of Mg ions from the implant site. The solubility in water should preferably be lower than 10 g/L and more preferably lower than 1 g/L.